KK vs possible flush

Party Poker No-Limit Hold'em, $ Hero (6 max, 6 handed) FTR converter on zerodivide.cx

UTG ($64.91)
MP ($24.35)
CO ($121.51)
Button ($64)
SB ($64.40)
Hero ($50)

Preflop: Hero is BB with K:spade:, K:club:. SB posts a blind of $0.25.
UTG calls $0.50, 1 fold, CO raises to $1, 2 folds, Hero raises to $3.5, UTG folds, CO calls $3.

Flop: ($8.75) 5:heart:, 8:club:, 9:club: (2 players)
Hero bets $8, CO calls $8.

Turn: ($24.75) A:diamond: (2 players)
Hero bets $15, CO calls $15.

River: ($54.75) 2:club: (2 players)
Hero checks, CO bets $25, Hero folds.

Final Pot: $79.75


first hand at the table and this happens... should i have made a blocking bet? if yes how much? how could i have played it better?